1. Field of the Invention
This invention relates to a cellular telephone system, and more particularly to a carrier synchronizing unit for a coherent detecting data communication system over non-frequency selective fading channels.
2. Description of the Related Art
Coherent detection schemes are superior compared to differential coherent or noncoherent schemes in terms of power efficiency. However, carrier recovery required for coherent detection is influenced by a time-varying characteristic of a fading channel. The power efficiency presented by coherent detection in a digital communication system is effective only when a carrier synchronizing unit is provided for a receiver.
In order to obtain a good result in carrier synchronism, it is required to accurately estimate complicated fading distortion.
Rapid fading is a central subject to be solved in digital mobile communication. For convenience of mounting or because of the lack of robust phase estimation algorithms, a differential detection technique or some other noncoherent technique has been traditionally used in a fading channel. A considerable improvement in performance can be achieved if near coherent demodulation is realized.
A linear demodulation system which employs coherent reception such as multi-phase shift keying (MPSK) or multi-quadrature amplitude modulation (M-QAM) potentially forms a preferable communication scheme.
The advantage in power efficiency of coherent detection over noncoherent detection really arises when channel coding or cochannel interference is taken into consideration.
When the channel is distorted by Rayleigh fading and the channel phase varies rapidly, an efficient carrier synchronizing technique wherein a phase is extracted from a received signal should be used for a coherent demodulation system.
Employment of an adaptive algorithm based on a recursive least square method (which is be hereinafter referred to simply as RLS) for estimation of a multiplicative distortion of a fading channel, equivalent to estimation of a phase and an amplitude of a carrier, improves the accuracy in estimation, reduces the influence of additive noise upon the estimated value and enhances the reliability of the estimated value. However, in order that variations in amplitude and phase caused by modulation of a carrier in order to transmit data may be identified from distortions by fading, the modulation must be removed.
This indicates that a decision-directed carrier synchronization architecture is required. Decision-directed processing (a method wherein, using a result of a decision, next processing is performed) has a hang-up phenomenon similar to that of a phase locked loop (PLL), and is furthermore restricted in that only a causal phase and amplitude estimation method can be used.
An exemplary one of many methods of non-causal phase and amplitude estimation is smoothing processing. The smoothing method improves the phase and amplitude estimation performance since a reference estimated value at the current point of time is produced using a value of a multiplicative distortion of a signal which is received later in time. However, such a signal later in time cannot be used in the decision-directed carrier synchronization architecture.
The weighting coefficient for an adaptive algorithm based on the recursive least square method must have a value close to "1" in order to minimize the influence of noise in an estimated value of the multiplicative transmission line distortion.
As the weighting coefficient increases, the influence of the additive noise decreases, but the response speed to the variation in multiplicative distortion decreases. As the weighting coefficient increases, the convergence of an RLS estimator decreases, which gives rise to a tracking delay in estimation. The tracking delay is disruptive in the decision-directed carrier synchronization architecture and leads to a burst error.
If both components (a real number component and an imaginary number component) of a multiplicative distortion become small at the same time (a base band equivalent at fading distortion is represented as a multiplication by time-varying complex number values), the amplitude is decreased significantly and a large variation in phase is caused.
Such rapid phase variation causes difficulties in a phase tracking system such as a decision-directed synchronization system. Furthermore, the tracking delay gives rise to errors in symbol decision and further deteriorates the estimation as the thus produced wrong symbols are used for estimation, which leads to a phase error and hang-up of the carrier synchronizing unit.
A combination of predictions based on a least squares fading memory fitting (curve fitting) and extrapolation and the recursive least square method brings about improvement in carrier tracking capability and minimizes the hang-up phenomenon.
Indeed, the hang-up phenomenon is minimized by such a method as described above, but it is impossible to completely eliminate the hang-up phenomenon.
In contrast, a method of interpolating pilot symbols can be readily realized. A receiver interpolates a channel measurement value provided by pilot symbols in order to obtain phase and amplitude references for detection.
However, ordinary symbol interpolation such as linear interpolation, interpolation by means of a lowpass filter or Gaussian interpolation has redundancy, and unless a pilot symbol rate is increased, the multiplicative distortion of a fading channel cannot be accurately estimated.
In order to achieve optimum interpolation, the Winer filter has been proposed. The Winer filter is a linear filter which has a load function which minimizes the square mean value of an error e(t) between an aimed value and the output. With the Winer filter, although the redundancy can be reduced, a large amount of calculation is still required.